Improve first Slack response after app installation

## Issue

When a user installs the Slack app, the first response message should be more helpful and guide users to run the dev server.

## Recommendation

Update the first Slack response to:

```
Congrats! Your app is now installed! Run bun run dev in your terminal to continue chatting with your Slackbot. When you're ready to share it with your team, you can deploy it using blink deploy.
```

## Context

This might help someone avoid a frustrating experience if they're chatting with the Slackbot when `dev` isn't running.